1. Field of the Invention
The present invention refers to a control circuit and an associated method of controlling the output voltage of a primarily controlled switching power supply. The switching power supply comprises a primary-sided switch and a transformer with an auxiliary winding, in which after opening the primary-sided switch an auxiliary voltage reflecting the output voltage is induced. The voltage induced in the auxiliary winding is supplied to a control circuit as control variable. The present invention further refers to an associated switching power supply.
2. Description of the Related Art
Usually, known switching power supplies use a power transistor as a switch to supply a pulsed current flow to a network of inductive and capacitive energy storage elements which convert the switched current pulses into a controlled direct voltage. Switching power supplies may supply output voltages that are greater, equal to or of opposite polarity as the uncontrolled input voltage, depending on the operating mode of the switching power supply. Switching power supplies are often used in power supply circuits. It is particularly desirable that such a switching power supply accepts input voltages in the range of 85 V alternating voltage to 270 V alternating voltage and thus may operate with different network supplies all over the world without modifications or switches.
The output voltage of such a switching power supply is usually controlled by means of a feedback signal which reflects the output voltage. This feedback signal is used to control the working cycle of the switching power transistor. There are different approaches to provide a suitable feedback signal. An auxiliary winding may for instance be provided, which during the switch-off time of the primary switch generates a feedback signal which supplies a reflection of the output voltage.
Switching power supplies of this type with auxiliary windings are for instance shown in the German Patent application DE 103 10 361, the European patent application EP 03 016 065.9, the U.S. Pat. No. 5,438,499 or the German laying-open specification DE 197 11 771 A1. The signal generated in the auxiliary winding is supplied to a feedback circuit which supplies the control signal to the control circuit.
In the case of a switching power supply operating according to the blocking converter type, in which the transmitted energy per pulse is equal and the duration of the breaks between the pulses is adjustable, as is shown in EP 03 016 065.9, the output voltage may very well be reflected and controlled with the primary auxiliary voltage.
In the above-mentioned switching power supplies the problem occurs that the output current can be detected in a very laborious way only. The power flow time of the secondary winding of the converter can for instance be determined, as shown in DE 103 10 361. As an alternative, an opto-coupler may be used, as it is for instance shown in the European patent application EP 1 146 630 A2.
In order to adjust the output voltage and the output current in a possibly simple and cost-effective manner and at the same time to minimize the component costs necessary for this purpose, a method is therefore suggested according to the German patent application DE 10 2004 016 927.6 in which the switching frequency of the primary-sided switch is adjusted depending on the auxiliary voltage in a manner that the output voltage and the output current of the switching power supply adopt values according to a predetermined output characteristics. This is based on the fact that in the case of a known output voltage the power required for a predetermined output current can be calculated according to the following equation
                    P        =                                            U              out                        ·                          I              out                                η                                    (        1        )            wherein P means the output power, Uout means the output voltage, Iout means the output current and η means the efficiency. Furthermore, the following connection formulated in the following equation 2 applies for the switching frequency f to be adjusted:
                    f        =                  P          W                                    (        2        )            with
      W    =                            L          prim                ·                  I                      prim            ,            max                    2                    2        ,wherein Lprim designates the primary-sided inductivity and Iprim,max designates the maximal primary-sided current.
If equation (1) is inserted into equation (2), the following results:
                                                        f              =                                                U                  out                                ·                                  I                  out                                ·                                  1                                                            η                      ·                      W                                        ︸                                                                                                                          =                              const                .                                                                        (        3        )            
However, this means that in the case of constant and known efficiency η the product of output voltage Uout and output current Iout linearly depends on the frequency f. Thus, the required switching frequency can directly be determined from the output voltage Uout fed back via the auxiliary voltage for a predetermined output current Iout. The advantage of the solution shown in DE 10 2004 016 927.6 is that laborious component-intensive measuring methods for determining the output current do no longer have to be provided. However, a component with five external terminals and the external components Ct and Rt are still required for the control circuit shown in this reference. An additional input is required for the detection of the current by the primary-sided switch. This increases the costs, the complexity and also the required space for the control circuit.